This invention relates generally to amplifiers, and more specifically to amplifiers for amplifying bi-directional signals.
A communication system 100, such as a two-way cable television system, is depicted in FIG. 1. The communication system 100 includes headend equipment 105 for generating forward signals that are transmitted in the forward, or downstream, direction along a communication medium, such as a fiber optic cable 110, to an optical node 115 that converts optical signals to radio frequency (RF) signals. The RF signals are further transmitted along another communication medium, such as coaxial cable 120, and are amplified, as necessary, by one or more distribution amplifiers 125 positioned along the communication medium. Taps 130 included in the cable television system split off portions of the forward signals for provision to subscriber equipment 135, such as set top terminals, computers, and televisions. In a two-way system, the subscriber equipment 135 can also generate reverse signals that are transmitted upstream, amplified by any distribution amplifiers 125, converted to optical signals, and provided to the headend equipment 105.
In a two-way system, an amplifier 125 typically incorporates diplex filters and gain blocks when the amplifier both receives and transmits signals, i.e., when the amplifier processes both forward, downstream signals and reverse, upstream signals. Diplex filters are three-port passive devices, each incorporating a high-pass filter and a low-pass filter for filtering the signals. Amplifiers 125 also include gain blocks for amplifying the signals, as illustrated in FIG. 2. The diplex filter 202 of the amplifier 125 has an input/output terminal 205 for processing bi-directional signals. More specifically, the input/output terminal 205 receives an input signal in the forward, or downstream, direction, and the input/output terminal 205 transmits an output signal in the reverse, or upstream, direction. The diplex filter 202 also includes an output port 210 for providing the forward signal in the downstream direction and an input port 215 for receiving the reverse signal from the upstream direction. The high-pass filter 220 and low-pass filter 225 of the diplex filter 202 provide isolation between the bi-directional signals.
Continuing to refer to FIG. 2, the output port 210 of the downstream diplex filter 202 transmits a filtered forward signal to a forward gain block 230 that amplifies the signal. The forward signal is then filtered through another diplex filter 238, which includes a high-pass filter 240 for transmitting the amplified forward signal to an input/output terminal 245 of the amplifier 125 and of the diplex filter 238. The input port 215 of the diplex filter 202 receives an amplified signal from a reverse gain block 235. The reverse signal has been filtered through a low-pass filter 250 of the diplex filter 238. This reverse signal is transmitted upstream, within the reverse path of the amplifier 125, from the input/output terminal 245 of the amplifier 125. Traditionally, amplifiers utilize a distinct gain block, as shown, for the forward path and a separate gain block for reverse path to preserve isolation between forward and reverse signals.
Conventionally, the reverse path signal is routed through the low-pass filters 225, 250 in the two diplex filters 202, 238 due to the frequency of the reverse path being in the low-band frequency spectrum that is typically 5 MHz to 40 MHz. The forward path is routed through the high-pass filters 220, 240 in the two diplex filters 202, 238 due to the frequency of the forward path being in the forward frequency spectrum that is typically 52 MHz to 870 MHz.
It will be appreciated, however, that there are variations of the frequency bandwidth values in the low-band and forward-band depending upon the preferences of the cable television system. The conventional amplifier 125 does have the ability to process different forward and reverse splits by utilizing a plug-in diplex filter. The signals are then routed through the respective gain block for amplification of the forward or reverse signal. More specifically, an operator may, for example, choose to use the reverse bandwidth of 5 MHz to 65 MHz and the forward bandwidth of 88 MHz to 750 MHz. Generally, however, the bandwidth splits are rarely changed once decided due to the potential impact on the channel programming in the lower end of the forward bandwidth. For example, if a television program channel is established at a low-end frequency in the forward bandwidth and the reverse bandwidth is increased, thereby changing the bandwidth split, the program channel quality at the low-end will be negatively impacted or eliminated.
There is, however, an increased demand for reverse services, which requires a corresponding increase in reverse bandwidth. As a result of the increased requirements and in light of the difficulties of increasing the conventional reverse bandwidth, cable television systems may, in the future, begin utilizing a higher-band frequency spectrum for the reverse path, in addition to the low-band frequency spectrum. This higher frequency spectrum may, for instance, be in the 900 MHz to 1200 MHz range. If this is done, reverse bandwidth may be increased without impacting the lower frequency channels in the forward bandwidth.
It will be appreciated, however, that installing new amplifiers with an additional gain block to accommodate this higher-band frequency for the reverse path could substantially increase the cost, the complexity, and the size of the amplifiers. Thus, what is needed is an improved amplifier that can better process forward and reverse signals in any bandwidth split in the downstream and upstream paths.